Epidemiological studies suggest that the sulfate and nitrate particulate fraction of urban aerosols are associated with acute human health effects. Before chemical and particle size specific air quality standards for particulate matter can be established and proper control strategies can be developed, more acute dose-response data under well- controlled conditions are needed to identify and characterize the specific irritants in the smog aerosol. This study proposes to determine the relative toxicity and the acute dose-response of guinea pigs and squirrel monkeys during inhalation of selected sulfate, nitrate and condensed hydrocarbon aerosols. Acute dose-response is measured in terms of the increase in flow resistance at fixed conditions of particle diameter, particle concentration and relative humidity over the ranges of these parameters found in urban atmospheres. These data will be analyzed in novel ways to test whether "hot spots" of particle deposition and the rate of dissolution of soluble irritants are important parameters in the acute dose-response mechanism. By employing lung modeling procedures, a standard methodology will be developed for scaling the acute dosage, and possibly the acute response of irritants in animals to man. The last phase of the investigation will test the feasibility of quantitating "hot spots" of irritant aerosols in vivo following acute exposures.